The isotope measurement consists of determining the presence and/or concentration in a sample of one or more isotopes of a chemical element.
It is performed in particular in the nuclear, life science (bioavailability, speciation studies, etc.), environmental and earth science (determination of isotope variation, speciation and migration-retention of elements, etc.) fields.
The isotope measurement is commonly performed using an ICPMS spectrometer.
This type of spectrometer consists of a plasma torch and a mass spectrometer.
The plasma torch contains a gas that, under the action of an electrical discharge and a radiofrequency field, generates a plasma that ionizes, with close to 100% efficiency, all or some of the elements introduced into to the torch in elemental or compound form.
The ions thus formed are then analyzed by the mass spectrometer portion that detects each ion according to its mass-to-charge ratio.
ICPMS spectrometry has been an indispensable analysis technique for many years. It makes it possible to quickly analyze, qualitatively and quantitatively, at least 70 elements of the periodic table, with good repeatability, sensitivity, resolution and a linear relationship between the quantity of species to be analyzed and the signal detected.
When the solution to be analyzed comprises multiple charged species, it is, however, preferable to improve the resolution of the measurement by first separating the species with a separation technique such as chromatography or capillary zone electrophoresis.
This separation upstream of the measurement is essential in particular when the solution to be analyzed contains species having an isobaric interference due to their adjacent mass. This is the case, for example, of elements 150Nd and 150Sm, with respective atomic masses of 149.920887 and 149.917271 atomic mass units (u).
In practice, the separation technique can be associated with the ICPMS spectrometer by an indirect (or off-line) coupling or by a direct (or on-line) coupling mode.
In indirect coupling, the isotope measurement is performed in two stages. In a first stage, the species contained in the solution to be analyzed are separated, and then collected individually at the outlet of the separation technique. In a second stage, each collected fraction is dried by heating, nitric acid is added, and then the collected fraction is analyzed by the ICPMS spectrometer.
Each fraction contains a single species. It therefore has a homogeneous composition. The signal measured by the ICPMS spectrometer is then continuous, which has the advantage of ensuring the stability and therefore the precision of the measurement.
Indirect coupling nevertheless has the disadvantage of requiring fraction collection and treatment steps, which are difficult to automate, and which significantly prolong the overall time of the analysis process.
In direct coupling, the isotope measurement is performed in a single sequence. Once separated, the species are introduced continuously into the ICPMS spectrometer coupled to the separation technique via a suitable interface. The direct coupling therefore eliminates the collected fraction treatment inherent to indirect coupling, and thereby notably reduces the measurement time. The document “Pitois A. et al, international Journal Of Mass Spectrometry, 2008, 270, pages 118-126” thus proposes an isotope measurement in which nuclear fission products are separated by zone electrophoresis using a capillary electrophoresis device connected by direct coupling to an ICPMS spectrometer.
Direct coupling nevertheless has a disadvantage in that the composition varies over time as the elution zone specific to each separated species reaches the outlet of the separation technique. This causes a significant and rapid variation in the signal between the measurement points. This transient signal is den recorded by the ICPMS spectrometer with a precision and reproducibility inferior to that of indirect coupling.
Moreover, it appears to be difficult to estimate the extent of the elution zone over which it is desirable to perform the recording by the ICPMS spectrometer, which is detrimental to the representativeness of the measurement. Thus, the reproducibility in direct coupling is generally ten times worse than that obtained in indirect coupling.